The present invention relates generally to nuclear reactors and, more particularly, is concerned with a trap mounted below the bottom grid and above the bottom nozzle of a fuel assembly and adjacent the lower end plugs of the fuel rods supported in the fuel assembly for trapping debris left in the reactor after assembly, repair and/or replacement operations so as to prevent entry and lodging thereof in the fuel assembly bottom grid where the debris can cause cladding perforations in the fuel rods and other damage to the fuel assembly. 2. Description of the Prior Art
During manufacture and subsequent installation and repair of components comprising a nuclear reactor coolant circulation system, diligent effort is made to held assure removal of all debris from the reactor vessel and its associated systems which circulate coolant therethrough under various operating conditions. Although elaborate procedures are carried out to help assure debris removal, experience shows that in spite of the safeguards used to effect such removal, some chips and metal particles still remain hidden in the system.
In particular, fuel assembly damage due to debris trapped at the bottom grid has been noted in several reactors in recent years. The damage consists of fuel rod tube perforations caused by fretting of debris in contact with the exterior of the tube. The debris tends to be relatively thin rectangular pieces, as opposed to pieces which are spherical in shape. Specifically, most of the debris consists of metal turnings which were probably left in the primary system after steam generator repair or replacement. The debris lodges in the region of the lower most grid within the spaces between its "egg-crate" shaped cell walls and the lower end portion of the fuel rod tubes. Amost all of the debris is deposited in the bottom grid just above the four coolant flow openings in the lower core support plate.
One approach has been to provide longer end plugs on the lower ends of the fuel rods which extend upwardly through the bottom part of the lowermost grid so that any fretting would occur against the solid end plug and not the cladding of the fuel rod tube. Although effective in reducing fuel rod cladding perforation, this concept has the disadvantage that the fuel stack height is shortened by up to several inches which has adverse effects on margins and on achieving longer burnups.
Several other different approaches have been proposed and tried for carrying out removal of debris from nuclear reactors. Many of these approaches, involving the use of screens at various places in the reactor core, of basins for catching the debris or of removable filters during preoperational testing, are discussed in U.S. Pat. No. 4,096,032 to Mayers et al.
While all of the approaches referred to above operate reasonably well and generally achieve their objectives under the range of operating conditions for which they were designed, they also create several other problems. Consequently, a need still exists for a fresh approach to the problem of debris capture in nuclear reactors. The new approach must be compatible with the existing structure and operation of the components of the reactor, be effective throughout the operating cycle of the reactor, and at least provides overall benefits which outweigh the costs it adds to the reactor.